The name is Portuguese. Bateira means a small boat, and the word came into Osaka speech through the long period of contact between Japan and Portuguese traders.

Around 1893, a sushi shop in Junkeimachi in Osaka put a fillet of konoshiro onto a bed of sushi rice, and the shape of the result apparently resembled a little boat. So the thing was called battera, and for whatever reason the name stayed on it.

The method was then applied to mackerel instead, and over time the name detached itself from the original fish altogether. What battera now means is vinegar-cured mackerel pressed down onto rice in a wooden mold, with a sheet of pale shiraita kombu laid over the top of it, and the whole block cut into rectangles.

So nothing that gave the thing its name in the first place is still present in it. Not the konoshiro. Not even the boat shape, which has been replaced entirely by the straight sides that a wooden mold produces. What survived is a Portuguese word for a rowing boat, attached to a squared-off block of pressed mackerel in a box in a shop in Osaka. Names routinely outlive the things they originally described, and usually nobody notices that it has happened.